Modern pulp and paper mills invariably have computerized control systems for monitoring and controlling the manufacture of moving webs of products, such as webs of paper or paperboard. In addition to ensuring that the paper and/or cardboard machines perform as desired and produce product of the appropriate quality, the control systems operate the machines as efficiently as possible. In order to accomplish this, the controllers associated with each machine gather large volumes of data regarding performance of the machine and the quality of the paper product so that the data can be analyzed.
With the increasing cost of energy and materials needed to run the machines and produce the paper, mill owners are continuously looking for ways to manage and conserve both energy and materials. Paper that is produced outside of the quality specifications set by the customer typically results in excess consumption of energy or materials, or both. For example, portions of the product having basis weight and/or moisture that is out of specification must often be rejected. Although the rejected portions can be re-pulped and used in subsequent productions, the energy used to produce the rejected portions cannot be recaptured. Even if no portions of the product are rejected, portions having higher basis weight than specified represent increased manufacturing costs due to the extra fiber that is used. Similarly, portions of the product having increased moisture may force the moisture target for the entire reel to be lowered, which results in increased manufacturing costs due to the extra steam needed to dry the reel. Accordingly, manufacturing a reel outside the quality specifications invariably results in lost or missing profit opportunities.
Numerous methods are known in the prior art for regulating the quality parameters (e.g., basis weight and moisture) of a paper or paperboard web in an effort to produce product within the desired quality range. According to one known method, measurements of the quality parameters are obtained at the dry end of the machine and compared to predetermined target values to obtain variances or “error” values that are used to maintain the parameters within desired control bands. For example, the variances in basis weight can be used to vary the concentration of material suspension supplied to the headbox at the wet-end of the machine to regulate the basis weight of the web being produced. Similarly, the variances in moisture can be used to vary the speed of the web so that the amount of time the web remains in the dryer section is increased or decreased as needed to regulate the moisture of the web being produced. Alternatively, or in addition, the variances in moisture can be used to alter the amount of heat energy (e.g., steam or gas) supplied to the dryer section to increase or decrease the evaporation rate.
Due to the time lag between when the measurements of the quality parameters are taken and the corresponding adjustments in machine operation are made, periodic variations having relatively long wavelengths (in the machine direction) will invariably result. However, additional and significant periodic variations in the quality variables can also result in the machine as well as cross-directions due to other factors such as temperature variations, pressure variations, finishing tolerances, and errors in the operation or adjustment of the machine. As a result, paper mills often run their machines at higher weights and lower moistures than requested by the customer to ensure that the lower tail of sampling distribution is above the specification limit. As problems with machines develop over time, however, the targets are often shifted further even away from the customer's specifications to compensate for poor machine operation, often without the mill being aware how much this shifted operation is costing in terms of additional materials, energy and lost production.
In nearly all modern process control systems, the measurements of the web parameters (e.g., basis weight and moisture) are transmitted to a database server and stored in a database program (called a “data historian”) for future use and analysis. In addition to basis weight and moisture content, the data historian may record other web quality parameters, such as caliper and opacity. In addition, the data historian may be used to record parameters associated with the operation of the machine such as bearing vibrations, steam and pump pressures, fan speeds, temperatures, and the like. As persons skilled in the art will appreciate, these machine parameters may be used to diagnose the condition of the machine when problems are suspected using techniques that are well known in the industry.
Although modern control systems are capable of monitoring and recording numerous parameters associated with quality of the web product and operation of the machine, this data is only as useful as the tools used for analyzing it. For example, it is known in the art to manually download data from the data historian into a spreadsheet and to hand calculate the histogram of the basis weight and moisture values. Based on the rule of thumb that 50% of the process variation can be removed by a good control system, it has been possible in the past to estimate the potential savings in fiber and energy by reducing the basis weight target and increasing the moisture target. However, this process is manually intensive and requires the services of a skilled technician to visually identify the variations that are potentially removable along with identifying the possible causes. As such, these calculations have heretofore been conducted only on an infrequent basis, such as when a machine is first commissioned or after a significant change has been made in machine operation.
In view of the foregoing, it would be beneficial to provide a fully automated system and method for estimating the economic cost of operating the machine to produce web product that is outside the quality specification called for by the customer. It would also be beneficial to provide a fully automated system and method for assessing this economic cost that is able to operate continuously and substantially in real time. In addition, it would be desirable to provide a system and method for verifying the economic benefits obtained after an improvement has been implemented in the process. Moreover, it would be beneficial to provide a system and method capable of automatically analyzing machine operation to identify potential improvements that could provide economic benefit.